1. Field of the Invention
The present invention relates to a stereoscopic image display device.
2. Related Art
A method for recording a stereoscopic image in any means and reproducing the recorded image as a stereoscopic image called “an integral photography method” (hereinafter, also called “an IP method”) or a ray reproducing method which displays many parallax images has been known. It is assumed that an object is viewed by the left and right eyes of an observer. An angle to A point formed by the left and right eyes when A point positioned near to the observer is viewed by his/her left and right eyes is defined as α, and an angle thereto formed by the left and right eyes when B point positioned far from the observer is viewed by his/her left and right eyes is defined as β, α and β vary in accordance with a positional relationship between the object and the observer. The (α-β) is called “binocular parallax”, and a person is sensitive to the binocular parallax so that he/she can see an object stereoscopically.
In recent years, development of stereoscopic displays of a spectacleless type has been advanced. Though a two-dimensional display is ordinarily used in most of these displays, an angle of rays from a display can be controlled as if the rays are emitted from an object positioned at a distance of several centimeters from the display when the object is viewed from an observer by disposing any ray control element on a front face or a back face of the display and utilizing the previously described binocular parallax. Even if rays of the display are distributed to several kinds of angles (called “parallax”) due to high fineness of the display, an image with a high fineness to a certain extent can be obtained on a background.
An outline of a constitution of a display of the spectacleless type stereoscopic displays which gives a parallax in a horizontal direction will be explained. The display is provided with an optical element between a two-dimensional image display device and an observer. A large number of image information obtained when an object is viewed in certain observation directions have been displayed on the two-dimensional display device, so that a stereoscopic image is displayed according to an observation direction by the observer observing an image through an optical plate (a ray control element) which is provided on a front face of a display plane and has opening portions such as slits, pin holes, micro-lenses, a lenticular or the like and a shield portion. Since this stereoscopic display allows a multi-parallax display, even if an observer moves, he/she can view an image corresponding to his/her position. That is, since display of a motion parallax is made possible, natural stereoscopic view is possible. Further, rays reproducing a stereoscopic image trace a route similar to one in a case that a real object has been disposed actually, so that the display performance is excellent in a point that a problem about visual field conflict does not arise.
Now, as a method for producing a parallax image to display the parallax image as respective pixel information through an opening portion, there are roughly two kinds of a method for performing an image mapping by generating rays reproducing a stereoscopic image from a pixel side and a method for performing an image mapping by tracing rays from a view point position of an observer toward pixels in a reverse manner. Here, both the methods are discriminated by calling an image mapping using the former method an IP method and calling the latter method a multi-parallax stereoscope parallax barrier method.
Ray bundles in the IP method are not emitted toward a position of the eyes of an observer but they are emitted from all the opening portions toward the observer at about equal intervals by the number of parallaxes. Therefore, since the IP method is superior in movement parallax when an observer has moved, the number of constituent pixels at an angle where a view point position has been fixed is reduced as compared with a display for an original two-dimensional displaying and a resolution is deteriorated as compared with a stereoscopic display which emits rays toward the positions of the eyes of an observer. In case that any constant resolution is required for a character display or a spherical display having an oblique component to a lens or a slit, in a three-dimensional display for parallel projection which is formed with a lenticular (a slit on a flat plane)+a display device, the resolution is restricted by a pitch of the lenticular, so that it has been difficult to perform a fine character expression or a smooth curve display. Examples devised so as to display a two-dimensional character or a two-dimensional image as a three-dimensional display are as follows:
In an image display method using a lenticular lens, an image display method which can clarify a display position of a stereoscopic image and can set a depth of the stereoscopic image arbitrarily has been proposed (for example, refer to JP07-49466A). In this method, each image of original images on n faces is reduced to a size of 1/n thereof in a direction of enlargement conducted by a lenticular lens, the reduced image is divided into stripe-like images with a width of about 1/n of the width of the lens pitch P of the lenticular lens, and the stripe-like images are sequentially distributed to each unit lens of the lenticular lens so that a composite image is formed. Simultaneously, the composite image is arranged at a predetermined distance from the lenticular lens on the basis of the magnitude of the lenticular lens such that the composite image is enlarged about n times and imaged on a predetermined imaging face through the lenticular lens, so that a display position of a stereoscopic image can be clarified and the depth of the stereoscopic image can be set to a predetermined value by providing a desired burr among the original images on the n faces. The method described in JP07-49466A describes a method for producing a character on an arbitrary display position, but does not describe improvement in character resolution.
Next, a telop display device which is constituted so as to telop-display a character or the like in a digital stereoscopic broadcasting which compresses and processes video signals for left eye and right eye to transmit them, where a telop such as an emergency broadcasting or the like can be displayed on a screen without injuring a stereoscopic feeling of a stereoscopic video image while a stereoscopic broadcasting program is being provided has been provided (for example, refer to JP10-327430A). The telop display device is constituted so as to receive a digital broadcasting at a tuner through an antenna to separate the same into video data, telop data and sound data at a separating circuit. When such a judgement is made by a CPU that data is the telop data, a telop data presence/absence display circuit is lightened on and an viewers sees the lighting to provide a command from a remote controller. Thereby, a video display portion which is displaying a stereoscopic video image based upon a video image for a right eye and a video image for a left eye is switched so that telop information added with a parallax is displayed on a video image display portion. The telop display device describes a circuit system but does not describe an optimal position for a character display.
Finally, an image display device which can display a two-dimensional image on a three-dimensional display device which allows display of a three-dimensional image has been known (for example, refer to JP2001-333437A). The image display device is provided with a three-dimensional display device and a control section which controls the three-dimensional display device. The control section acquires a rendering pattern corresponding to 2-dimensional image data to display the rendering pattern on the three-dimensional display device. The rendering pattern is constituted such that colors obtained when the rendering pattern has been displayed on the three-dimensional display device become artificially equal to colors obtained when two-dimensional image data has been displayed on a two-dimensional display device. The image display device is directed to improvement in image mapping in order to solve such a problem that, when arrangement of parallax images is performed with sub-pixels of R, G and B in a horizontal direction, characters are colored, but it does not describe any improvement in resolution.
In the IP method, when a solid is reproduced at a position separated from a display plane, there arises a problem that a resolution lowers urgently due to spreading of a ray bundle assigned through an opening portion or a lens (for example, refer to a non-patented literature 1 (H. Hoshino, F. Okano, H. Isono and I. Yuyama “Analysis of resolution limitation of integral photography” J. Opt. Soc. Am, A15 (1998) 2059-2065.)).
In the IP method, such a problem that, when a solid is reproduced at a position separated from a display plane, a resolution lowers urgently due to spreading of a ray bundle assigned through an opening portion or a lens will be explained below.
As a scale representing a resolution of a stereoscopic display, β (cycle per radian: cpr) is used. β is an index indicating the number of cycles of a contrast which can be displayed per radian. As illustrated in FIG. 20, in an IP method, a resolution βnyq at a stereoscopic image in the vicinity of a display is called Nyquist frequency, and it is determined depending on a distance from an observer to an opening portion and a pixel pitch viewed through a lens. When an opening portion pitch is represented by pe and a distance from an observer to an opening portion or a lens is represented by L, the resolution βnyq limited by the opening portion pitch pe is expressed as follows:βnyq=L/(2pe)  (1)
Next, as illustrated in FIG. 21, when an object 73 is reproduced at a position separated from a display plane, namely at a position separated from an observer 64 by a distance zi, a ray bundle 67 assigned through opening portions 62 or lenses of an optical element 68 is spread so that a resolution is lowered urgently. When the object 73 is reproduced on a region protruded from the display device 61 or on a region in a depth of thereof, the maximum value of a resolution computed from ray group emitted from one slit in order to reproduce an image of the object is represented by αimax, a spatial frequency of the object viewed from an observation point is expressed as follows:βimax=αimax×zi/|L−zi|  (2)
Incidentally, L is a distance between the observer 64 and the ray control element 68. Since an actual resolution is a lower one of the above equations (1) and (2), the following equation is adopted.Bimax=min(βimax, βnyq)   (3)
Here, it will be understood from the equation (1) that according to reduction of the opening portion pitch pe, namely higher fineness of the display plane, the resolution of the stereoscopic image increases. However, there arises a problem that narrowing a pixel pitch of the display plane itself causes a process change or the like so that such a narrowing can not be realized easily. In this connection, in FIG. 21, the ray control element 68 is a slit, and comprises opening portions 62 and shielding portions 63.
Further, in case that there is the stereoscopic image 73 in the vicinity of the display plane, since βnyq becomes smaller than βimax, the resolution of βnyq is predominate. Furthermore, as the stereoscopic image 73 is separated from the display plane, since the value of zi in the equation (2) becomes smaller, so that the resolution of the βimax becomes predominate. For example, a resolution determined from the equations (1) and (2) regarding the number of parallaxes and the viewing area angle is illustrated in FIG. 18. In FIG. 18, a horizontal axis z is a distance from an observer to a stereoscopic display object 73, and a display 61 is disposed at a position of Z=1.5 m. A vertical axis indicates a resolution βnyq determined by a lens pitch determined according to the equation (1) and a resolution βimax determined from a ray density emitted from one lens in a lenticular lens determined according to the equation (2). From FIG. 18, it is understood that since the resolution βnyq determined from the lens pitch becomes smaller than the resolution βimax regarding an object positioned in the vicinity of a display plane, namely, expressed in a range defined by a projection amount zn=0.12 m and a depth amount zf=0.13 m, the resolution βnyq becomes predominate, while the resolution βimax becomes predominate in a region where the projection amount is larger than zn and in a region where the depth amount determined depending on a ray density from the opening portion is larger than zf.
From FIG. 18, it is understood that such a problem arises that, since display can not be made at a resolution equal to or more than the Nyquist frequency in a two-dimensional image requiring a high resolution, such as a character, a character with a relatively large size where the lens pitch is defined by one dot must be displayed.
Moreover, there is such a search result that, when the number of constituent dots for a character is 12×12 or more, a character becomes easy to see (for example, refer to TOSHIBA REVIEW Vol. 57 No. 6(2002) “Human engineering research regarding reading easiness of character in highly fine LCD” by Kenji IDO and etc.). Therefore, assuming that the lens pitch is 1.5 mm, when a 12 point character is displayed, one character size becomes 1.5×12=60 mm, so that the character size becomes considerably large, which results in reduction of the number of characters which can be displayed.
FIG. 19A illustrates a ray locus and a constitution of a three-dimensional (stereoscopic) display device which has been viewed from the above. In FIG. 19A, the three-dimensional display device is illustrated to have a two-dimensional display device 1, an optical element 2 such as a lenticular lens or the like, and one pupil 4 of an observer. It is assumed that a display position of a two-dimensional character which attains the maximum resolution is defined on the two-dimensional display device 1. A two-dimensional character displayed on the two-dimensional display device 1 is shown in FIG. 19B. In FIG. 19A, as aspect of main rays 7 of beans to be originally incident on the pupil 4 which is directed to an observer exactly opposed to a front face of the display device 1 is illustrated. For example, in case that the number of parallaxes is 11, as illustrated in FIG. 19C, only the ray 7 of 6 parallaxes at a center enters in the pupil 6. Therefore, since an image obtained by only the main ray 7 entering in the pupil 4 spreads according to the lens width, as illustrated in FIG. 19D, a resolution becomes insufficient, which results in collapse of a character. Similarly, even when the lenticular lens 2 is replaced with a slit, the number of sampling points is reduced, so that it becomes difficult to recognize a character.